Transition state imbalance in proton transfer from phenyl ring-substituted2-tetralones to acetate ion

Rate constants for the acetate ion-catalyzed ketonization of phenyl-substit uted 2-tetralone enols have been determined by stopped-flow UV spectroscopy . From these rate constants and the keto-enol equilibrium constants, the ra te constants (k(-2)) for enolization were calculated. A Bronsted plot of th ese rate constants (log k(-2)) vs the acidity of the appropriate 2-tetralon e (pK(a)(K)) is linear, with a slope (-alpha(E)) of -0.78 +/- 0.03, except for the point Corresponding to 6-nitro-2-tetralone (4b). Rate constants for the ionization of 2-tetralone by substituted acetates were;determined dire ctly by NMR, giving a corresponding Bronsted beta(E) of 0.54 +/- 0.03. Both the negative deviation of the-point for 4b from the correlation line for a lpha(E) and the inequality between alpha(E) and beta(E) indicate an imbalan ced transition state for the proton abstraction of 2-tetralone by acetate i on. This reaction is impeded by a thermodynamic barrier of 11 kcal/mol, alo ng with an intrinsic kinetic barrier of 14 kcal/mol. A comparison of the tr ansition states for proton abstraction of 2-tetralone by hydroxide ion and by acetate ion shows similar transition state imbalance and intrinsic kinet ic, barriers for both reactions. The relevance of these results to the mech anism of enzymatic acceleration of enolization is discussed.